When an automobile is running at a high speed, collisions of small stones or the like against plates (strictly speaking, film surfaces) of the automobile are unavoidable. There are cases where such collisions of small stones sprung by motor traffic against films form cracks on the films or peel off the film from the substrates such as outside plates or other parts (so-called chipping is generated). If the chipping is generated on the film, water, etc., is permeated through the resulting flaw to form rust on the surface of the steel substrate.
In particular, in such regions as the northern part of the U.S., Canada, and northern Europe, where rock salt and sand are scattered in large quantities on roads for the purpose of melting snow in the winter season, it is particularly important for the films on steel substrates of automotive body to have chipping resistance. Thus, it is desired to obtain a film which, even when small stones collide against it, does not suffer damages (chipping) and protects the steel substrate against rusting.
The coating of steel substrater of automotive body is generally conducted by a method in which a steel plate that has been chemical-treated with iron or zinc phosphate is formed successively with an electrodeposition coatings (i.e., primer), an intermediate-coating paint, and a topcoating paint. In order to improve chipping resistance and rust-preventive properties, various improvements have so far been proposed on electrodeposition coatings (i.e., primer), intermediate-coating paints (i.e., intermediate coatings), and topcoating paint and on coating methods therefor.
For example, U.S. Pat. No. 3,900,126 (corresponding to JP-B-58-49583; the term "JP-B" as used herein means an "examined Japanese patent publication") discloses a metallic can having a glued layer on the seam portion, which is characterized in that the glued layer comprises an olefin copolymer having a carboxyl group in the side chain and a polyurethane resin in a proportion of 5:100 to 100:5 by weight, the olefin copolymer being the copolymer of an aliphatic .alpha.-olefin having not more than 10 carbon atoms with an .alpha.,.beta.-ethylenically unsaturated carboxylic acid having 3 to 8 carbon atoms, which has a melt index of 30 to 300 dg/min, and the polyurethane resin being the reaction product of an isocyanate with either a polyether or a polyester.
JP-A-61-242668 discloses a coating method which comprises applying electrodeposition coatings on a steel, subsequently applying thereon a water-based barrier coating (paint) which forms a film having a static glass transition temperature of 0.degree. to -75.degree. C., and then applying thereon an intermediate-coating (paint) and a topcoating (paint). Preferred examples of the resin for the water-soluble or water-dispersible barrier-coating (paint) to be used in this method include a modified polyolefin resin, styrene-butadiene copolymer, butadiene resin, and acrylic resin. The specific examples of the modified polyolefin resin which are given in the reference include a mixture obtained by blending a propylene-ethylene copolymer with a chlorinated polyolefin and a graft polymer obtained by graft-polymerizing maleic acid or anhydride with a propylene-ethylene copolymer.
Further, GB 2,249,497 (corresponding to JP-A-4-122474) discloses a method for coating an outside plate of automotive body comprising a metallic member and a plastic member. In this method, a metallic member coated with cationic electrodeposition coatings and a plastic member are assembled beforehand into an automotive body part, and this body part is coated with a water-soluble or water-dispersible barrier-coating paint containing as the major vehicle component a composition comprising an olefin resin and a urethane resin. The resulting body part may be coated with an intermediate-coating paint, if necessary, and is then coated with a topcoating paint.
The examples of the olefin resin for use in the above water-soluble or water-dispersible barrier-coating paint which are given in GB 2,249,497 (corresponding to JP-A-4-122474) include a propylene-ethylene copolymer, chlorinated polypropylene, EPDM (ethylene-propylene-diene copolymer), polybutadiene, styrene-butadiene copolymer, acrylonitrile-butadiene copolymer, and acid radical-containing polymers obtained by polymerizing these polymers with an acid radical-containing, polymerizable unsaturated monomer such as maleic acid, maleic anhydride, acrylic acid, or methacrylic acid. In this reference, there are descriptions to the effects that an acid radical-containing polymer obtained by polymerizing maleic acid (anhydride) with a chlorinated polypropylene is especially preferred, and that in the case of using maleic acid, the amount of maleic acid (anhydride) incorporated in the acid radical-containing polymer is from 0.5 to 10% by weight based on the amount of the polymer.
However, the methods described above are still unsatisfactory in sufficient chipping resistance. It has been ascertained from investigations made by the present inventors that even when a water-based chipping primer is prepared by dissolving or dispersing a conventional chipping-resistant primer of the solvent-based type by changing the polymerization method for the conventional primer or using a surfactant or emulsifying agent, the chipping-resistant film obtained from this water-soluble or water-dispersible chipping primer cannot sufficiently absorb the impact energy of collisions of small stones or the like against the film and, hence, sufficient chipping resistance cannot be obtained.